A high voltage bushing (“HVB”) can be used to conduct electric current from a stator winding connection of a generator through the generator frame to a buss external to the generator. In a gas-cooled generator (e.g., a hydrogen-cooled generator), the HVB isolates voltage from the generator frame and provides a gas tight connection. On higher rated generators, direct gas-cooled HVBs are used along with other electrical conductors and/or components to transmit the electric current from the winding to the external buss. In these generators, cooling gas flows through a hollow HVB conductor and a hollow tubular lead. Physical and/or electrical degradation of these and other electrical components can occur due to heat, vibration, or other factors, many of which occur during the course of normal use and aging. Degradation and/or failure of the electrical components can lead to overheating of the HVB, causing degradation and/or failure of the HVB. HVB degradation and/or failure can include, but is not limited to, degradation and/or failure of gas seal(s)/gaskets within the HVB which causes the HVB to leak hydrogen and/or corona resistant filler (e.g., asphalt), aging of the HVB flange bond, and electrical degradation and/or failure. HVB degradation and/or failure can necessitate HVB maintenance, repair, or replacement.
Routine inspections and maintenance can be scheduled, during which the need to repair or replace an HVB can be detected. Leaking asphalt is one possible indicator of a failing HVB. Asphalt can be used to fill air space in the HVB connection to prevent undesirable electrical discharge (e.g. arcing between components). The air-side (exposed side) of the HVB is an area where evidence of asphalt leaking is likely to be discovered. In the event the seals fail and asphalt leaks from the HVB, it is normally recommended that the HVB be rebuilt or replaced with a new HVB, to protect from hydrogen leakage.
HVB degradation and/or failure can also be indicated by in-service gas loss, in gas-cooled generators. Further, an HVB replacement can be prompted by generator uprating, resulting in the need for higher rated HVBs, or by other factors not associated with degradation, damage, or failure of the HVBs or other electrical components.
An HVB and/or an HVB connection can be repaired and/or replaced by disassembling and reassembling it. Before disassembly, a first winding resistance on each phase can be recorded to obtain baseline data. The disassembly of the HVB connection can be performed by removing insulation and putty between the HVB and the lower stud connector, unbolting the connection between the HVB, the lower stud connector, and a tubular lead, and removing the lower stud connector to free the HVB to be removed.
The tubular lead and the lower stud connector can be visually inspected for overheating and their silver plating integrity. The plating can be restored if desired. Usually, there is no evidence of overheating on the HVB conductor, the lower stud connector, or the tubular lead.
The reassembly of the HVB connection can be performed by the reverse of the disassembly process, except that the lower stud connector bolts can be replaced with new stainless steel hardware. Once the lower stud connector bolts are properly torqued, a second winding resistance can be recorded, and the connection can be re-insulated.
Attempts to repair and/or replace HVBs using current methods have resulted in ensuing failures of the HVBs. These failures have occurred either immediately after the maintenance, or relatively soon thereafter, in a time significantly less than the expected or average life of an HVB. For example, recent failures have surfaced over the past 1 to 3 years with replacement HVB's installed by more than one service provider. In each case, evidence of leaking asphalt was detected during inspections. Accordingly, current industry practices to repair and/or replace HVBs are insufficient in addressing the HVB failure and/or its cause.
In some cases of failure after repair or reassembly of an HVB the winding resistance was recorded, and the results were consistent with past maintenance outage measurements, and in both cases, measurements were consistent between phases. The results would appear to indicate that the HVB connections were acceptable per the industry acceptance criteria e.g., max. of 2% differential between phases when compared to one another). Accordingly, current industry practices to detect HVB connection deficiencies and/or HVB failure causes are insufficient.
It would be advantageous to perform maintenance, repair, or replacement of an HVB to avoid, prevent, or reduce the chance of, an ensuing failure that occurs sooner than the expected life of an HVB.